The present disclosure relates to a battery cell and to a method of operating the same.
Lithium ion batteries have now become widespread in products such as cellular phones and electric vehicles. A lithium ion battery usually comprises a cathode and an anode between which a separator comprising a carbonate-based electrolyte is located. In a first charging of the battery, lithium ions are liberated by the cathode, migrate through the separator and are stored in the material of the anode. During discharge, a reverse flow of the lithium ions is established.
Over a prolonged period of operation with numerous charging and discharging cycles of the lithium ion battery, short circuits attributable to the formation of lithium dendrites or metallic impurities which affect both the anode and the cathode can occur in the battery. Such short circuits lead to a rapid increase in the internal battery temperature to temperatures greater than 200° C.-250° C. If the thermal energy liberated here exceeds a particular limit, the carbonate-based electrolyte decomposes and produces large volumes of predominantly toxic gases. These large amounts of toxic gases are given off into the surroundings of the battery in a relatively short time or the battery explodes as a result of the excessive internal pressure.
To avoid such short circuits, lithium ion batteries of this type normally comprise at least one layer composed of a metal oxide such as aluminum oxide or titanium dioxide which is positioned on the large area of the anode or the cathode. This layer is referred to as safety function layer (SFL) or as heat retardant layer (HRL). It is electrically nonconductive and thus increases the internal, electrical resistance of the battery and adversely affects the electrochemistry within the lithium ion cell. Furthermore, it increases the weight of the battery.
Thermally expandable graphite intercalation compounds are known from a different context, for example as fire protection materials. Thus, such compounds are disclosed, for example, in EP 1489136 A1, WO 2011/039301 A1 and EP 1323670 A2. The production of expanded graphite is disclosed, for example, in DE 102007053652 A1 and the production of expandable graphite intercalation compounds is described, for example, by the book N. N Queenwood and A. Earlshore, “Chemistry of the elements” Second Edition, Elsevier Butterworf-Heinemann 2005, p. 293 to p. 296.